1. Field of the Invention
The present invention relates to an optical add/drop multiplexer for dropping and adding an optical signal to and from a wavelength-division multiplexed optical signal and passing the wavelength-division multiplexed optical signal in which input light having constant optical power is input to an optical amplifier in the optical add/drop multiplexer in spite of a variation in the number of channels of the wavelength-division multiplexed optical signal. The invention also relates to an optical amplifying apparatus used in such an optical add/drop multiplexer as well as to an optical amplifying method.
At present, ultra-long distance, large-capacity optical transmission systems are required to construct future multimedia networks. As one scheme for attaining such increase in capacity, the wavelength-division multiplexing (hereinafter abbreviated as WDM) is now being studied and developed because of its advantage that the wide bandwidth and the large capacity of the optical fiber can be utilized effectively.
In particular, in recent years, it has come to be required to realize not only optical transmission systems in which a WDM optical signal is received and transmitted between two end stations but also optical transmission systems having a network function such as an ADM (add-drop multiplexer) function. The ADM function is a function of selectively passing only optical signals having particular wavelengths of a WDM optical signal at a repeater station called a node that is provided at an intermediate position on an optical transmission line, dropping optical signals having wavelength excluding the above particular wavelengths at the node, or sending other optical signals that are added at the node to another node. In the above circumstances, the optical add/drop multiplexer (hereinafter abbreviated as OADM) having the ADM function that is a key device of optical transmission systems is now being studied extensively.
2. Description of the Related Art
In recent years, the optical amplifier capable of amplifying an optical signal as it is without converting it into an electrical signal is used in optical transmission systems.
In the optical amplifier, by supplying energy to a laser medium, electrons in the laser medium are excited and population inversion is formed there. If input light is input to the laser medium in a state that population inversion is formed there, the input light causes stimulated emission and is thereby amplified.
For example, the optical fiber amplifier amplifies input light in such a manner that electrons of a rare earth element are given population inversion by supplying pump light to an rare-earth-ion-doped optical fiber. The semiconductor optical amplifier amplifies input light in such a manner that electron population inversion is formed by supplying an injection current (driving current) to a semiconductor active layer such as a pn junction.
FIG. 10 shows how the gain varies as the optical power of input light varies in a case where the excitation energy for forming population inversion is kept constant.
In FIG. 10, the horizontal axis represents the optical power of input light to an optical amplifier and the vertical axis represents the gain of the optical amplifier. FIG. 10 shows a case of large excitation energy and a case where the excitation energy is smaller than in the former case.
As shown in FIG. 10, where the excitation energy is kept constant, when the optical power of input light is increased the gain is kept constant in a certain range of the optical power. However, when the optical power of input light is increased beyond the above range, the gain decreases gradually. This is due to the following reason.
The number of electrons at upper level is constant because the excitation energy is constant. Therefore, when the optical power of input light is small, electrons at upper level that are consumed by stimulated emission exist in a sufficient number and hence a constant gain is obtained. However, when the optical power of input light is large, the number of electrons at upper level is insufficient and hence the gain lowers.
For example, this phenomenon is described on page 12 of “Optical Amplifier and Its Applications” (supervised by Hideki Ishio, Ohmsha, Ltd.) and page 209 of “Optical fiber Communication Technology” (supervised by Yoshihiro Konishi, The Nikkan Kogyo Shinbun, Ltd.).
On the other hand, it is known that the gain-wavelength characteristic of the optical amplifier varies with the wavelength of input light.
In the optical amplifier, spectral-hole burning occurs which is a phenomenon that when input light having large optical power concentrated in a single wavelength is input, the gain at the wavelength of the input light becomes smaller than in a case where the input light is absent or smaller.
Therefore, in the case of using the optical amplifier, to obtain a prescribed gain, it is necessary to determine an operating point of the optical amplifier in consideration of the optical power of input light and other factors.
Incidentally, in an optical communication system having an OADM, optical signals (channels) are added to or dropped from a WDM optical signal in the OADM and hence the optical power of the WDM optical signal varies. Therefore, the optical power of input light varies in an optical amplifier for amplifying the WDM optical signal in the OADM.
In particular, the optical power of input light varies in an optical amplifier for amplifying a WDM optical signal that is input from an optical transmission line to the OADM, because the number of channels increases or decreases in an OADM disposed upstream of the OADM concerned.
An optical amplifier for amplifying a WDM optical signal consisting of optical signals (channels) that pass through the OADM concerned has a problem that the optical power of input light varies because an arbitrary number of optical signals (channels) are dropped by a dropping unit in the OADM.
Further, in an optical amplifier for amplifying a WDM optical signal to be output of the OADM concerned, the optical power of input light varies because an arbitrary number of optical signals (channels) are added to a WDM optical signal that pass through the OADM by an adding unit in the OADM.